The inception of Darwin's theory occurred during an intensively busy period which began when Charles Darwin returned from the survey voyage of the Beagle, with his reputation as a fossil collector and geologist already established. He was given an allowance from his father to become a gentleman naturalist rather than a clergyman, and his first tasks were to find suitable experts to describe his collections, write out his Journal and Remarks, and present papers on his findings to the Geological Society of London. At Darwin's geological début, the anatomist Richard Owen's reports on the fossils showed that extinct species were related to current species in the same locality, and the ornithologist John Gould showed that bird specimens from the Galápagos Islands were of distinct species related to places, not just varieties. These points convinced Darwin that transmutation of species must be occurring, and in his Red Notebook he jotted down his first evolutionary ideas. He began specific transmutation notebooks with speculations on variation in offspring "to adapt & alter the race to changing world", and sketched an "irregularly branched" genealogical branching of a single evolutionary tree. Animal observations of an orangutan at the zoo showed how human its expressions looked, confirming his thoughts from the Beagle voyage that there was little gulf between man and animals. He investigated animal breeding and found parallels to nature removing runts and keeping the fit, with farmers deliberately selecting breeding animals so that through "a thousand intermediate forms" their descendants were significantly changed. His speculations on instincts and mental traits suggested habits, beliefs and facial expressions having evolved, and considered the social implications. While this was his "prime hobby", he was struggling with an immense workload and began suffering from his illness. Having taken a break from work, his thoughts of marriage turned to his cousin Emma Wedgwood. Reading about Malthus and natural law led him to apply to his search the Malthusian logic of social thinking of struggle for survival with no handouts, and he "had at last got a theory by which to work". He proposed to Emma and was accepted. In his theory, he compared breeders selecting traits to natural selection from variants thrown up by "chance", and continued to look to the countryside for supporting information. On 24 January 1839 he was elected as Fellow of the Royal Society, and on the 29th married Emma. The development of Darwin's theory followed.
Background Darwin was not the first to propose that species of organisms could become modified over time. In the third edition of On the Origin of Species Darwin provided a historical sketch of his predecessors in writing of descent with modification or natural selection, including those whom he had only learned of after the 1859 publication of The Origin. His account essentially deals with 19th-century authors; "Passing over authors from the classical period to that of Buffon, with whose writings I am not familiar, Lamarck was the first man whose conclusions on this subject excited much attention." However, in a footnote he remarks on how his grandfather, Dr. Erasmus Darwin, Goethe and Geoffroy Saint Hilaire came to the same conclusion on the origin of species in the years 1794–95, anticipating Lamarck. After his early life in a Unitarian family, Charles Darwin developed his interest in natural history. At Edinburgh University his work as a student of Robert Edmund Grant involved him in pioneering investigations of the ideas of Lamarck and Erasmus Darwin on homology showing common descent, but he also saw how controversial and troubling such theories were. Robert Jameson's course taught Darwin stratigraphic geology, and closed with lectures on the "Origin of the Species of Animals". At Christ's College, Cambridge to qualify as an Anglican clergyman, Darwin became passionate about beetle collecting, then shone in John Stevens Henslow's botany course. He was convinced by Paley's Natural Theology which set out the Teleological argument that complexity of "design" in nature proved God's role as Creator, and by the views of Paley and John Herschel that creation was by laws which science could discover, not by intermittent miracles. The geology course of Adam Sedgwick and summer work mapping strata in Wales emphasised that life on earth went back over eons of time. Then on his voyage on the Beagle Darwin became convinced by Charles Lyell's uniformitarian theory of gradual geological process (see Lyell's Principles of Geology, which Darwin took along on the Beagle), and puzzled over how various theories of creation fitted the evidence he saw. However, Darwin did not conceive the theory of evolution during the Beagle's voyage, later recounting that:
it was equally evident that neither the action of the surrounding conditions, nor the will of the organisms (especially in the case of plants), could account for the innumerable cases in which organisms of every kind are beautifully adapted to their habitats of life—for instance, a woodpecker or a tree-frog to climb trees, or a seed for dispersal by hooks or plumes. I had always been much struck by such adaptations, and until these could be explained it seemed to me almost useless to endeavour to prove by indirect evidence that species have been modified.
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